Sculpt hard edges mouth
I'm not very much into sculpting, but you can try the Layers tool. Make sure you are in Subtraction Mode (the minus-button on the top) and set the falloff type to Constant.
But to make the edges so smooth like in your reference, you would need a hella lot of vertices, which can be performance-eating.
This is the closest result i achieved with sculpting lol:

You could try it with Bump Maps. I have to admit I haven't done this myself, but i've seen a colleague doing this. Basically you use an image with black an white values (black = low, white = high) as your source for the heights.
I think you would have to paint your mouth in grey tones and a sharp edge on a white background and plug in certain nodes in the shader editor.
I found a pretty short, but on-point-tutorial on this, which i can recommend (warning, it may be a bit fast): https://www.youtube.com/watch?v=_dU3DONwSzU